1. Field of the Invention
The present invention relates to an electric parking brake system in which a parking brake is operated by an electric motor.
Further, the present invention also relates to an electric parking brake system for operating and releasing parking brakes provided on left and right rear wheels by an electric motor.
2. Discussion of Background Art
Electric parking brake systems are known in which an electric motor is driven to operate or release a parking brake when the operation of an associated switch is detected or the stop of a vehicle is detected, and the electric parking brake systems of this kind are sometimes used as a back-up system in case a service brake which is operated by a brake pedal fails (for example, refer to the following patent literature).
[Patent Literature]
JP-B-4-28576
Incidentally, while the brake force of a general electric parking brake system is controlled based on the current value of an electric motor and the amount of stroke of a cable which operates a parking brake, a brake force actually obtained varies depending on the conditions of a friction material (for example, the temperature and degree of wear of the friction material) when the parking brake is applied even if the current value and the stroke amount are controlled to remain the same. Consequently, there is caused a problem that a deceleration (a braking force) of the vehicle obtained when the driver operates the operating member such as a manual operation command switch to operate electric parking brakes while the vehicle is running varies from time to time, and moreover, there is also caused a problem that unlike the service brakes, the driver cannot adjust the magnitude of braking force of the electric parking brakes arbitrarily.
Further, in the event that the electric parking brake system is actuated when the vehicle is running on a road surface having a low friction coefficient, when the braking force increases excessively relative to the road surface friction coefficient, there is caused a possibility that the wheels are locked, and, in particular, when the rear wheels are locked, there may be caused a risk that the stability in behaviors of the vehicle is deteriorated. On the other hand, when attempting to use the parking brake system as a back-up system for the service brake, a certain magnitude of braking force is required, and when attempting to meet the requirement, the braking force tends to increase excessively, whereby there is caused a problem that the rear wheels are easily locked.
To cope with the problem, while it is considered that the upper limit value of the braking force of the electric parking brake system is restricted based on the current value of an electric motor and the stroke amount of a cable used to operate the parking brake, even if the current value and the stroke amount are controlled to remain the same, a braking force that is actually obtained varies depending on the conditions of a friction material when the brake is applied (for example, the temperature and degree of wear of the friction material), and therefore, it is difficult to accurately regulate the upper limit of the braking force of the electric parking brake system.
Further, the invention is made in view of the situations, and an object thereof is to prevent the locking of the rear wheels in an ensured fashion without being affected by the conditions of the friction materials when the brakes are applied to the rear wheels of a running vehicle by the electric parking brake system.